1. Field of the Invention
The present invention relates to a semiconductor laser device which emits laser light having a wavelength of 0.7 to 1.2 xcexcm.
2. Description of the Related Art
In many conventional semiconductor laser devices which emit laser light having a wavelength of 0.7 to 1.2 xcexcm, a current confinement structure and an index-guided structure are provided in crystal layers constituting each semiconductor laser device so that each semiconductor laser device oscillates in a fundamental transverse mode.
For example, J. K. Wade et al. (xe2x80x9c6.1 W continuous wave front-facet power from Al-free active-region (xcex=805 nm) diode lasers,xe2x80x9d Applied Physics Letters, vol. 72, No. 1, 1998, pp.4-6) disclose a semiconductor laser device which emits light in the 805 nm band. The semiconductor laser device comprises an Al-free InGaAsP active layer, an InGaP optical waveguide layer, and InAlGaP cladding layers. In addition, in order to improve the characteristics in the high output power range, the semiconductor laser device includes a so-called large optical cavity (LOC) structure in which the thickness of the optical waveguide layer is increased so as to reduce the peak power density, and increase the maximum optical output power. However, when the optical power is maximized, currents generated by optical absorption in the vicinity of end faces generate heat, i.e., raise the temperature at the end faces. In addition, the raised temperature reduces the band gaps at the end faces, and therefore the optical absorption is further enhanced to damage the end face. That is, a vicious cycle is formed. This damage is the so-called catastrophic optical mirror damage (COMD). When the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. Therefore, the above semiconductor laser device is not reliable when the semiconductor laser device operates with high output power.
Further, T. Fukunaga et al. (xe2x80x9cHighly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 xcexcm Separate Confinement Heterostructure Lasers,xe2x80x9d Japanese Journal of Applied Physics, vol. 34 (1995) L1175-L1177) disclose a semiconductor laser device which comprises an Al-free active layer, and emits light in the 0.8 xcexcm band. In the semiconductor laser device, an n-type AlGaAs cladding layer, an intrinsic (i-type) InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-type InGaP optical waveguide layer, a p-type AlGaAs cladding layer, and a p-type GaAs cap layer are formed on an n-type GaAs substrate. However, the maximum optical output power of the semiconductor laser device is typically 1.8 W, i.e., low.
As explained above, the conventional semiconductor laser devices which emit laser light in the 0.8 xcexcm band are not reliable when the semiconductor laser device operates with high output power since the catastrophic optical mirror damage or the like occurs.
The object of the present invention is to provide a semiconductor laser device which emits laser light having a wavelength in the range of 0.7 to 1.2 xcexcm, and is reliable even when the semiconductor laser device operates with high output power.
According to the present invention, there is provided a first semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer made of InGaP of an undoped type or the first conductive type, and formed on the lower cladding layer; an active layer made of InGaAsP or InGaAs, and formed on the lower optical waveguide layer except for near-edge areas of the lower optical waveguide layer which are adjacent to opposite end faces of the semiconductor laser device, where the opposite end faces are perpendicular to a direction of laser light which oscillates in the semiconductor laser device; a first upper optical waveguide layer made of InGaP of an undoped type or a second conductive type, and formed on the active layer; a second upper optical waveguide layer made of InGaP of an undoped type or the second conductive type, and formed over the first upper optical waveguide layer and the near-edge areas of the lower optical waveguide layer; an upper cladding layer of the second conductive type, formed on the second upper optical waveguide layer; and a contact layer of the second conductive type, formed on the upper cladding layer.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (vi).
(i) In the semiconductor laser device, a ridge structure may be formed by removing more than one portion of the upper cladding layer and the contact layer, and a bottom of the ridge structure may have a width of 1.5 xcexcm or more.
(ii) The semiconductor laser device may further include an additional layer made of InGaAlP of the first conductive type, and formed on the second upper optical waveguide layer other than a stripe area of the second upper optical waveguide layer so as to form a stripe groove realizing a current injection window, the upper cladding layer may be formed over the additional layer so as to fill the stripe groove, and a bottom of the stripe groove may have a width of 1.5 xcexcm or more.
(iii) The active layer may be made of Inx1Ga1-x1As1-y1Py1 where 0xe2x89xa6x1xe2x89xa60.3, 0xe2x89xa6y1xe2x89xa60.5, and the product of the strain and the thickness of the active layer may be in a range of xe2x88x920.15 to +0.15 nm.
The strain D of a layer formed on the GaAs substrate is defined as D=(cxe2x88x92cs)/cs, where cs and c are the lattice constants of the GaAs substrate and the layer formed on the GaAs substrate, respectively
(iv) The active layer may be a strained quantum well active layer, at least one barrier layer made of InGaP may be formed adjacent to the strained quantum well active layer, the at least one barrier layer may be oppositely strained to the strained quantum well active layer, and the sum of a first product and a second product may be in a range of xe2x88x920.15 to +0.15 nm, where the first product is the product of the strain and the thickness of the active layer, and the second product is the product of the strain and the total thickness of the at least one barrier layer.
(v) Each of the lower cladding layer and the upper cladding layer may be made of Alz1Ga1-z1As, or Inx3(Alz3Ga1-z3)1-x3As1-y3Py3, where 0.55xe2x89xa6z1xe2x89xa60.8, x3=0.49y3xc2x10.01, 0 less than y3xe2x89xa61, and 0 less than z3xe2x89xa61.
(vi) Each of the lower optical waveguide layer and the first upper optical waveguide layers may be made of Inx2Ga1-x2P, where x2=0.49xc2x10.01.
According to the present invention, there is provided a second semiconductor laser device comprising: a GaAs substrate of a first conductive type; and a semiconductor layer formed on the GaAs substrate, the semiconductor layer including: a cladding layer of a first conductive type, formed on the GaAs substrate; a lower optical waveguide layer made of InGaP of the first conductive type or an undoped type, the lower optical wavelength layer being formed on the lower cladding layer; a compressive strain active layer made of InGaAsP or InGaAs, the compressive strain active layer being formed on the lower optical waveguide layer; an upper optical waveguide layer made of InGaP of a second conductive type or an undoped type, the upper optical waveguide layer being formed on the compressive strain active layer; and a cladding layer of the second conductive type.
Here, the second semiconductor laser device of the present invention is characterized in that: an InGaAsP lower barrier layer is provided between the lower optical waveguide layer and the compressive strain active layer, the InGaAsP lower barrier layer having a band gap larger than that of the compressive strain active layer; an InGaAsP upper barrier layer is provided between the compressive strain active layer and the upper optical waveguide layer, the InGaAsP lower barrier layer having a band gap larger than that of the compressive strain active layer; portions of the lower barrier layer, the compressive strain active layer and the upper barrier layer are removed, which are adjacent to two opposite end faces constituting a resonator end face among end faces obtained by cleaving the semiconductor layer; the lower and upper optical waveguide layers have band gaps larger than that of the compressive strain active layer; and the upper optical waveguide layer is buried in the removed portions of the lower barrier layer, the compressive strain active layer and the upper barrier layer.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (vii).
(i) The compressive strain active layer should be made of Inx3Ga1-x3As1-y3Py3, where 0.49y3 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1. Note that when y3=0 is satisfied, InGaAs containing no phosphorus is used.
(ii) The InGaAsP lower and upper barrier layers may have a compressive strain and a stretch strain, and the InGaAsP lower and upper barrier layers may be lattice-matched to each other. Note that an absolute value of a sum of products of strain quantities and film thickness of the InGaAsP lower and upper barrier layers shall be set to 0.3 nm or less.
(iii) A contact layer of the second conductive type should be formed in a region on the second cladding layer part, the region excluding a region corresponding to the portions where the lower barrier layer, the compressive strain active layer and the upper barrier layer are removed; and an insulating film should be formed in a region on the second cladding layer part, the region corresponding to the portions where the lower barrier layer, the compressive strain active layer and the upper barrier layer are removed.
(iv) A ridge portion may be provided, which is formed by removing both sides of a stripe-shaped portion of the cladding layer part of the second conductive type, the stripe-shaped portion extending from one resonator end face to the other resonator end face, from an upper surface of the cladding layer part of the second conductive type to a predetermined position.
(v) The cladding layer part of the second conductive type may be composed of one layer, or alternatively the cladding layer part may be composed of a plurality of layers For example, the cladding layer part of the second conductive type may comprise a first cladding layer of the second conductive type formed on the upper optical waveguide layer; a first etching stop layer made of InGaP of the second conductive type and being formed on the first cladding layer; a second etching stop layer made of GaAs having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the first etching stop layer; a current confinement layer made of InGaAlP of the first conductive type having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the second etching stop layer; a cap layer made of InGaP of the first conductive type having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the current confinement layer; and a second cladding layer of the second conductive type and being formed on the cap layer. Alternatively, the cladding layer part of the second conductive type may comprise an etching stop layer made of GaAs having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the upper optical waveguide layer; a current confinement layer of the first conductive type made of InGaAlP having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the etching stop layer; a cap layer of the first conductive type made of InGaP having a stripe-shaped current injection opening extending from one resonator end face to the other resonator end face and being formed on the current confinement layer; and a cladding layer of the second conductive type being formed on the cap layer.
(vi) In the second semiconductor laser device of the present invention, each of the cladding layers should be made of one of AlGaAs, InGaAlP and InGaAlAsP which are lattice-matched with the GaAs substrate.
(vii) The lower and upper barrier layers may be respectively composed of one layer of Inx1Ga1-x1As1-y1Py1 where 0xe2x89xa6x1xe2x89xa60.3 and 0xe2x89xa6y1xe2x89xa60.6, or composed of two layers of Inx2Ga1-x2As1-y2Py2 where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2 and a tensile strain barrier layer made of Inx4Ga1-x4As1-y4Py4 where 0.49y greater than x4xe2x89xa70 and 0 less than y4xe2x89xa60.5, and the tensile strain barrier layer is disposed so as to be adjacent to the compressive strain active layer.
A method of manufacturing a semiconductor laser device according to the present invention in which a plurality of semiconductor layers including a compressive strain active layer are laminated on a substrate and a resonator end face is constituted by two opposite end faces, comprises the steps of: forming a cladding layer of a first conductive type on a GaAs substrate of the first conductive type; forming an InGaP lower optical waveguide layer of the first conductive type or an undoped type on the cladding layer, the lower optical waveguide layer having a band gap larger than that of the compressive strain active layer; forming an InGaAsP lower barrier layer on the lower optical waveguide layer, the lower barrier layer having a band gap larger than that of the compressive strain active layer; forming the compressive strain active layer made of one of InGaAsP and InGaAs on the lower barrier layer; forming an InGaAsP upper barrier layer on the compressive strain active layer, the upper barrier layer having a band gap larger than that of the compressive strain active layer; forming an InGaP cap layer on the upper barrier layer; forming a GaAs cap layer on the cap layer; removing a portion of the GaAs cap layer in the vicinity of an end face adjacent to the resonator end face; removing a portion of the InGaP cap layer in the vicinity of the end face using the GaAs cap layer as a mask; removing the GaAs cap layer used as the mask and simultaneously removing portions in the vicinity of the end faces of the upper barrier layer, the compressive strain active layer and the lower barrier layer using the InGaP cap layer as a mask; forming an InGaP upper optical waveguide layer of a second conductive type or an undoped type having a band gap larger than that of the compressive strain active layer on the removed portions in the vicinity of the end face and the InGaP cap layer; and forming a cladding layer of the second conductive type on the upper optical waveguide layer.
Here, the cladding layer of the second conductive type is formed by the steps of: laminating a first cladding layer of the second conductive type on the upper optical waveguide layer; laminating a first etching stop layer made of InGaP of the second conductive type on the first cladding layer; laminating a second etching stop layer made of GaAs on the first etching stop layer; laminating an InGaAlP current confinement layer of the first conductive type on the second etching stop layer; laminating an InGaP cap layer of the first conductive type on the current confinement layer; laminating a second GaAs cap layer on the InGaP cap layer; removing a portion of the second GaAs cap layer corresponding to a stripe-shaped current injection opening; removing portions of the InGaP cap layer of the first conductive type and the current confinement layer, the portions being used as the current injection opening, using the second GaAs cap layer as a mask; removing the second GaAs cap layer used as the mask and simultaneously moving a portion of the second etching stop layer, the portion being used as the current injection opening, using the InGaP cap layer of the first conductive type as a mask; and forming a second cladding layer of the second conductive type so as to cover the current injection opening.
Alternatively, the cladding layer part of the second conductive type may be formed by the steps of: laminating a GaAs etching stop layer on the upper optical waveguide layer; laminating an InGaAlP current confinement layer of the first conductive type on the second GaAs etching stop layer; laminating an InGaP cap layer of the first conductive type on the current confinement layer; laminating a second GaAs cap layer on the InGaP cap layer; removing a portion of the second GaAs cap layer corresponding to a stripe-shaped current injection opening; removing portions of the InGaP cap layer of the first conductive type and the current confinement layer, the portions being used as the current injection opening, using the second GaAs cap layer as a mask; removing the second GaAs cap layer used as the mask and simultaneously removing a portion of the GaAs etching stop layer, the portion being used as the current injection opening, using the InGaP cap layer as a mask; and forming a cladding layer of the second conductive type so as to cover the current injection opening.
The first semiconductor laser devices according to the present invention have the following advantages.
In the semiconductor laser device according to the present invention, near-edge portions of the active layer and the first upper optical waveguide layer are removed, where the near-edge portions are adjacent to opposite end faces of the semiconductor laser device, and the opposite end faces are perpendicular to the direction of laser light which oscillates in the semiconductor laser device. In addition, the second upper optical waveguide layer is formed in the near-edge spaces from which the above near-edge portions of the active layer and the first upper optical waveguide layer are removed, and the second upper optical waveguide layer has a band gap greater than that of the active layer. That is, regions which are unabsorbent of (transparent to) the laser light oscillating in the semiconductor laser device are formed in the vicinity of the opposite end faces, and thus the aforementioned current generation caused by light absorption in the vicinity of the end faces can be prevented. Accordingly, the heat generation in the vicinity of the end faces during the high output power operation can be reduced, and therefore the catastrophic optical mirror damage (COMD) can be prevented, although, as explained before, the catastrophic optical mirror damage (COMD) occurs when the light absorption is enhanced by reduction of the band gap due to the heat generation at the end faces. Consequently, the optical output power of the semiconductor laser device according to the present invention can be greatly increased without the catastrophic optical mirror damage (COMD). That is, the semiconductor laser device according to the present invention is reliable even when the semiconductor laser device operates with high output power.
Further, when regions in the vicinity of opposite end faces of a semiconductor laser device having an internal-stripe type index-guided structure and an oscillation region with a width of 1.5 xcexcm or more, and oscillating in a fundamental transverse mode are made unabsorbent of (transparent to) laser light which oscillates in the semiconductor laser device, the semiconductor laser device is reliable even when the semiconductor laser device operates with high output power.
The second semiconductor laser device of the present invention adopts the structure (a so called window structure), in which the InGaAsP lower barrier layers having a band gap larger than that of the compressive strain active layer are provided between the lower optical waveguide layer and the compressive strain active layer as well as between the compressive strain active layer and the upper optical waveguide layer, the portions adjacent to the two opposite end faces that constitute the resonator end faces among the end faces obtained by cleaving the semiconductor layer are removed, and the upper optical waveguide layer having a band gap larger than that of the active layer is buried in the removed portions. Since a transparent region for oscillation light can be formed in the vicinity of the end face, it is possible to reduce current generated by light absorption at the end face. Thus, heat generation of the device at the end face can be decreased, and a breakdown level of the end face due to thermal runway generated in the end face can be significantly improved. Accordingly, the present invention can provide a semiconductor laser device with high reliability even when the semiconductor laser device is operated with high output power.
Furthermore, when the InGaAsP barrier layer is not provided between the InGaP optical waveguide layer and the InGaAsP compressive strain active layer, it would generally require a long time for switching gas from P to As, and substitution of P with As occurs at the interface, resulting in deterioration of the quality at the interface, that is, deterioration of the quality of the active layer. However, since the InGaAsP barrier layer is provided in the present invention, switching of gas can be performed smoothly, and the quality of the active layer can be improved. Note that if the InGaAsP barrier layer has tensile strain, there is an advantage that a temperature characteristic of the element can be improved.
The contact layer of the second conductive type is formed in a region on the cladding layer of the second conductive type, the region excluding a region corresponding to the portions where the lower barrier layer, the compressive strain active layer and the upper barrier layer are removed, and the insulating film is formed in a region on the second cladding layer, the region corresponding to the portions where the lower barrier layer, the compressive strain active layer and the upper barrier layer are removed. As a result, injection of current to the window region can be suppressed more effectively, and an increase in optical output power can be achieved.
By forming the ridge portion obtained by removing a portion of the cladding layer of the second conductive type and by constructing the current confinement structure in the cladding layer of the second conductive type as composed of a plurality of layers, the structure provided with a refractive index waveguide mechanism can be obtained. Thus, it is made possible to perform mode control of a laser beam with a high precision.
According to the method of manufacturing a semiconductor laser device of the present invention, when the plurality of semiconductor layers are laminated, and when the window structure and the current injection opening are formed, a GaAs cap layer is laminated on the InGaP cap layer serving as a re-growth surface afterwards, and a predetermined spot of a layer formed under the GaAs cap layer is removed using the GaAs cap layer as a mask. Thereafter, the GaAs cap layer is removed. By use of the above-described steps, it is made possible to prevent formation of a spontaneous oxide film over the InGaP cap layer and transmutation of the layer caused by formation of the resist layer directly thereon. Furthermore, by preventing adhesion of organic matter apt to remain on the re-growth surface, occurrence of crystal defects is controlled, and element characteristics and reliability of the device can be improved.